Add guards to the sysctl and ufw configurations for Ubuntu containers

- Ubuntu containers start with a base minimal base that doesn't contain
`ufw`. A guard was added to the task that disables `ufw` so that it
fails gracefully when `ufw` is not present.

- In addition, `/proc` is not writable. This prevents the plays in
this role from tuning kernel level settings via `sysctl`. Guards were
put in to ensure `/proc` is writable before attempting to apply new
settings with `sysctl`.